An experimental comparison of diffuser designs in a centrifugal compressor

The objective of this investigation was to compare the effects of two different diffuser vane geometries on the performance of a centrifugal compressor. Vaned diffusers are commonly used in aeroengines because they can achieve higher pressure recoveries than vaneless diffusers of similar size. Improving the diffuser’s pressure recovery and effectiveness can result in overall compressor performance benefits that are sought out by engine manufacturers looking to improve overall engine efficiency. An airfoil vane was tested and compared to a baseline wedge diffuser in the Purdue CSTAR research facility, where the centrifugal compressor used for this study is intended to be the last stage of an axial-centrifugal compressor. The overall stage performance for the compressor with the airfoil diffuser was first obtained at open impeller tip clearances and then at tight clearances. In the tight clearance configuration, the compressor consistently had higher total pressure ratio and higher isentropic efficiency compared to the open clearance configuration. In particular, impeller isentropic efficiency and shroud static pressure along the entire impeller passage were highest at the tight impeller clearance configuration, indicating less losses due to tip leakage flow. The reduction in tip leakage flow, and resulting blockage, at tight clearance operation was also suggested by the increase in choke mass flow rate for all corrected speeds. For the comparison of the airfoil and wedge diffuser assemblies, data were acquired with the impeller in the tight clearance configuration. Despite slight differences in exducer tip clearances (less than 2x10-3 in.), stage total pressure ratio and isentropic efficiency at 100% corrected speed were not significantly different between the two diffuser assemblies. Upon closer inspection of individual components, the airfoil diffuser actually had, on average, higher pressure recovery and higher diffuser effectiveness than the wedge diffuser. However, the airfoil diffuser assembly had higher losses in the deswirl region compared to the wedge diffuser assembly. Excluding measurements from the deswirl, the total pressure ratio for the airfoil diffuser assembly was slightly greater than that of the wedge diffuser assembly. A physical mismatch in the airfoil diffuser and deswirl assembly could be responsible for the losses recorded in the deswirl region and could have resulted in the limited performance improvements observed. Additionally, the similar vane geometries at the leading edge (in terms of inlet metal angle and thickness) combined with any deviation from design during manufacturing could have limited the predicted performance benefits